1. Field of the Invention
The present invention relates to a camera system, and more particularly, to a camera system in which a camera body and a unit are selectively mounted on the camera body and are driven by a supersonic motor perform operations for a camera in relation to each other by microcomputers or the like.
2. Related Art Statement
Recently, the advancement of the functions of the camera causes the microcomputer used in only the camera body to be provided as well in a unit which is detachable from the camera body. The microcomputer included by the unit and the microcomputer located in the camera body are arranged to be operated in synchronization with each other so theft the desired function of the camera system is accomplished. In order to efficiently control the unit in a structure of the above-mentioned type, a technological means for mutually communicating information between the two microcomputers by means of a serial communication has been disclosed in, for example, Japanese Patent Laid-Open No. (62-220937. Since the aforesaid technology must be established when the functions of a camera are performed while communicating a large quantity of information between the camera body and the unit, it has been widely used.
As the source for operating a driving mechanism included by the unit, a supersonic motor has been widely used recently. In this case, the driving frequency of the supersonic motor must be set to a range adjacent to the resonant frequency of the supersonic motor. However, each of the supersonic motors has a specific resonant frequency and therefore the driving frequency must be precisely adjusted in order to cause the motor to be satisfactorily rotated at a high efficiency.
For example, a technology for finely adjusting the oscillation frequency has been disclosed in Japanese Patent Laid-Open No. 62-234881 in which an analog type voltage controlling oscillation circuit having resistors and capacitors is used, feed-back voltage obtained by phase-detecting a feedback signal obtained from the feedback electrode of the supersonic motor and by smoothing it by a low-pass filter is applied to a control terminal of the voltage controlling oscillator so as to form a feedback loop.
The analog type oscillating circuit of the above-mentioned type has a problem in that the stability of the oscillation frequency deteriorates due to a factor such as the temperature. Furthermore, some means for changing the resistance value must be provided for the purpose of adjusting the circuit. Therefore, time and cost become too large as compared with a method in which the circuit is adjusted by a digital means using a memory or a microcomputer.
Accordingly, the applicant of the present invention has suggested a circuit for driving a supersonic motor in U.S. Pat. No. 5,013,982 which used an oscillating circuit for dividing the RF signal by a counter or the like in a digital manner, which reveals satisfactory controllability, which can be adjusted easily and from which a stable oscillation frequency can be obtained.
However, according to the disclosure made in U.S. Pat. No. 5,013,982 in which the frequency of the driving signal for driving the supersonic motor is obtained by dividing the RF signal by a counter or the like in a digital manner, the original oscillation frequency of the RF signal undesirably becomes EQU 50 kHz.times.(50 kHz/0.2 kHz)=12.5 MHz
in a case where the frequency is varied in units of, for example, 0.2 kHz, which is required to perform smooth control, assuming that the frequency for driving the motor is 50 kHz. What is worse, since a duty ratio of 50% cannot be realized in the above-described state in which the signal having the aforesaid original oscillation frequency is used, it is halved for securing safety. However, in this case, an original oscillation frequency of 25 MHz must be used. An oscillator adapted to the aforesaid high frequency and a digital circuit which is operated at the high frequency must be supplied with a large electric current of a value larger than 10 mA and as well as smaller than several tens of mA.
On the other hand, in order to establish automatic focusing and/or in order to raise the efficiency of the employed elements, a super-microcomputer capable of performing high speed calculation has been used in the camera body. For example, in a case where an oscillator adapted to 20 to 25 MHz and a microcomputer adapted to the aforesaid frequency are loaded on the camera body, a capacity (for example, 40 mA of the overall capacity 50 mA) larger than the half of that of the DC/DC converter which constitutes the IC-type power supply is undesirably consumed in the camera body. Therefore, a limit is present when the IC type power is supplied from the camera body to the unit. To the contrary, if the capacity of the DC/DC converter is enlarged to cope with the above-mentioned problem, the size of the camera becomes too large.
As a result, if the RF oscillator and the high frequency operation portion are individually provided in a unit in a case where the supersonic motor in the unit is driven by a driving circuit of a digital type, the power supply in the camera body must bear an excessively large load.
According to the method disclosed in U.S. Pat. No. 5,013,982, four types of digital signal must be used because its basic structure is adapted to the push-pull circuit. Furthermore, feedback signals are required to detect the phase of the signal for the monitor electrode of the supersonic motor and the velocity of the supersonic motor so as to feed back the frequency and the pulse width. If the above-described signals are transmitted/received between the unit and the camera body through an exclusive line, the number of signal pins increases excessively, causing limits to present in the location and the size of the contacts. As a result, the cost cannot be reduced and the signal transmission reliability at the contacts will deteriorate.
In Japanese Patent Laid-Open No. 61-251490 and U.S. Pat. No. 4,833,358, methods have been disclosed in which a feedback system for maintaining the phase constant for the purpose of stably driving the supersonic motor is provided, the method being arranged in such a manner that a monitor electrode is provided on the supersonic motor, the phase of the output signal from the monitor electrode with respect to the driving pulse is detected and the driving frequency is changed.
However, similar to U.S. Pat. No. 5,013,982, the aforesaid disclosures have not suggested a preferable method of separating the camera body and the unit from each other.
The applicant of the present invention has disclosed a means for driving the supersonic motor by employing a frequency oscillation of 4 n times. However, a plurality of signal transmission lines are formed from the camera body to the unit. Therefore, the number of signal pins increases excessively, causing limits to present in the location and the size of the contacts. As a result, the cost cannot be reduced and the signal transmission reliability at the contacts will deteriorate. Also the aforesaid disclosures have not suggested a preferable method of separating the camera body and the unit from each other.